1. Field of the Invention
The present invention relates to power inverter circuits for driving fluorescent lamps such as cold cathode fluorescent lamps (CCFLs) or the like. More specifically, the present invention relates to a split phase topology for power inverter circuits.
2. Description of the Related Art
Fluorescent lamps are used in a number of applications including, for example, backlighting for display screens, or the like. One particular type of fluorescent lamp is a cold cathode fluorescent lamp (CCFL). Such lamps require a high starting voltage (typically on the order of 700 to 1,600 volts) for a short period of time to ionize the gas contained within the lamp tubes and fire or ignite the lamp. This starting voltage may be referred to herein as a strike voltage or striking voltage. After the gas in a CCFL is ionized and the lamp is fired, less voltage is needed to keep the lamp on.
In liquid crystal display (LCD) applications, backlight is needed to illuminate the screen to make a visible display. Backlight systems in LCD or other applications typically include one or more lamps and an inverter system to provide DC to AC power conversion and to control the brightness of the lamps. Even brightness across the lamps and clean operation of inverters with low switching stresses, low EMI, and low switching losses are desirable. However, increases in lamp length, wire length and operating voltage associated with large backlighting systems make even luminance difficult to achieve. Even luminance across non-floating lamp structures is even more difficult. Thus, some backlight inverter systems are configured to support floating lamp structures.
The size of LCD display panels are increasing with the increasing size of large screen displays such as those associated with large screen TVs, desktop monitors, or the like. As the size of LCD display panels increase, the size of their backlighting systems also increase. The associated increase in power level of the backlight inverter systems exacerbates problems typically found in conventional half-bridge and push-pull inverter topologies. These problems may include, for example, switching spikes, high voltage/current stresses, switching losses, electromagnetic interference, combinations of the forgoing, or the like.
A number of conventional inverter topologies facilitate zero-voltage or zero current switching to reduce switching stresses and losses. These inverter topologies include, for example, an active clamping forward topology, a phase shifted full-bridge topology, a resonant full-bridge topology, an asymmetric half-bridge topology.
A factor in achieving even brightness over a lamp is the ability to symmetrically power the lamp at both ends. This is more difficult to achieve as the length of the lamp increases. Among the conventional inverter topologies, the phase shifted full-bridge topology and the resonant full-bridge topology are acceptable for CCFL inverter applications because of their ability to produce symmetric lamp current waveforms. However, there are some disadvantages associated with resonant type inverters including, for example, high amplitude of voltage or current excursion, variable operating frequency, or the like. These disadvantages are not desirable in many lamp applications.
FIG. 1 illustrates a conventional backlight system 100 for powering a lamp 102. The lamp 102 is coupled between the secondary winding of a first transformer 104 and the secondary winding of a second transformer 106. The primary winding of the first transformer 104 and the primary winding of the second transformer 106 are coupled to an switching network 110 through two switching power lines 112, 114. The switching network 110 comprises four power MOSFETS (metal oxide semiconductor field effect transistors) 120, 122, 124, 126 connected in a full-bridge topology to provide DC to AC conversion. The four power MOSFETS 120, 122, 124, 126 are coupled to DC power lines V+, V−. The disadvantage of the configuration shown in FIG. 1 is that high current or high voltage switching lines 112, 114 typically have to cross a display panel.
When the length of the lamp 102 increases with the panel size, the configuration shown in FIG. 2 may be used to avoid running long switching power lines across the length of the panel. The configuration shown in FIG. 2 powers a lamp 202 by using separate inverters 204, 206 at respective ends of the lamp 202. Both inverters 204, 206 use a full-bridge switching topology. Thus, as compared to the inverter shown in FIG. 1, the power devices and associated component count, including controllers 240, 242, is doubled. Increasing the number of components increases the cost and surface area of a printed circuit board using the product. Thus, the total size of the product is increased.